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Nanomesh pressure sensor preserves skin’s sense of touch

Researchers in Japan have developed the first artificial-skin patch that does not affect the touch sensitivity of the real skin beneath it. The new ultrathin sensor, which is made from multilayers of conductive and dielectric nanomesh structures, could be used in applications as diverse as prosthetics, robot-assisted surgery, human-machine interfaces and wearable health care monitors.

Skin is our largest sensory organ, with an abundance of neurons that continually monitor stimuli in our environment and transmit this information to the brain. A good artificial skin must replicate this ability. In particular, an electronic skin, or e-skin, needs to be highly sensitive to touch, while also responding quickly to applied pressure.

To achieve these goals, the e-skin needs to incorporate a high density of sensors, over areas at least as small as 50 microns. But they also need to avoid interfering with a person’s natural sense of touch, and this has proved difficult. Human fingertips, for example, are so sensitive that a piece of plastic foil just a few microns thick is enough to impair a person’s sensations.

“A wearable sensor for your fingers has to be extremely thin,” explains Sunghoon Lee, a member of Takao Someya’s group at the University of Tokyo and an author of the paper. “But this obviously makes it very fragile and susceptible to damage from rubbing or repeated physical actions.”

For this reason, Lee adds, most e-skins developed to date – made from, for example, sensor arrays of assembled nanowires or microstructure rubber layers that change capacitance or resistance in response to pressure or force – have been relatively thick and bulky.

Two layers

In contrast, the sensor developed by Someya, Lee and colleagues is thin and porous. It consists of two layers, both made using a process called electrospinning, and is based on a design proposed by Akihito Miyamoto and colleagues in 2017. The first layer is an insulating mesh-like network comprising polyurethane fibres around 200 to 400 nm thick. The second is a network of lines that makes up the functional electronic part of the device – a parallel-plate capacitor. This is made of gold on a supporting scaffold of polyvinyl alcohol (PVA), a water-soluble polymer often found in contact lenses. Once this layer has been fabricated, the researchers wash away the PVA to leave only the gold support. The finished pressure sensor is around 13 microns thick.

When a finger covered with this sensor grasps an object, the dielectric nanomesh layer deforms, producing a change in the capacitance measured between the two layers. When the researchers evaluated the device’s sensitivity as determined by the slope of the capacitance change-pressure curve, they found values (of 0.141 kPa^-1 in the low applied pressure range of less than 1 kPa and 0.010 kPa^-1 in the high applied pressure range of more than 10 kPa) that were comparable to the grip forces measured for a bare finger.

“We performed a rigorous set of tests on our sensors with the help of 18 test subjects,” Lee says. “They confirmed that the sensors were imperceptible and affected neither the ability to grip objects through friction, nor the perceived sensitivity compared to performing the same task without a sensor attached. This is exactly the result we were hoping for.”

Robust and resistant

As an added benefit, the researchers found that their nanomesh sensors continued to work even after being compressed repeatedly. Indeed, the devices’ capacitance changed by just 0.15% after they had been squeezed 1000 times at 19.6 kPa. The conductivity of the gold electrode also remained relatively stable during these experiments.

Electronic skin displays human-like reactions to pressure, temperature and pain

Another point in favour of the sensor is its resistance to friction: tests showed that it could be rubbed 300 times with a 50 g object (equivalent to applied pressures of more than 100 kPa) without breaking. The sensor’s electrical characteristics changed only slightly during these tests, and the device remained sensitive to applied pressures even after being rubbed.

The researchers, who report their work in Science, say they plan to increase the number of sensing points in their device and determine how pressure is spatially distributed across it. “We also hope to further develop other imperceptible sensors, such as temperature, strain and humidity sensors, to achieve multimodal sensing,” Lee tells Physics World.

New fuel gauge for spacecraft could keep satellites active for longer

When a spacecraft launches, it uses roughly 75–90% of its propellant getting into orbit. The remaining fraction determines how long it can remain up there, but gauging how much fuel is left in the tank is no easy task in zero gravity. Researchers at the US National Institute of Standards and Technology have now developed a solution based on a suite of sensors that detects the capacitance of liquid inside a spacecraft’s fuel tank and uses these data to reconstruct a three-dimensional picture of the remaining fuel. According to the team, the prototype design could enable satellites to operate for longer, while also helping to avoid damaging end-of-life collisions.

Under zero-gravity conditions, liquid propellants adhere to the inside of fuel tank walls due to surface tension and capillary effects. This unpredictable spatial distribution makes fuel levels hard to determine. Propellants are also free to slosh about, float and form bubbles – none of which happens on Earth.

Several techniques have been developed to measure onboard spacecraft propellant. One of the most common, known as the bookkeeping technique, involves estimating how much is burned with each thrust and subtracting this from the volume of fuel left in the tank. However, while this method is highly accurate at the beginning of a mission, the error of each estimate carries over to the next and accumulates with each thrust, explains team member Nick Dagalakis, a mechanical engineer. “By the time a tank is low, the estimates become more like rough guesses and can miss the mark by as much as 10%,” he says.

Without reliable fuel measurements, satellite operators are in a bind, Dagalakis adds. Retiring a satellite when it still has plenty of fuel left is a waste of money, but letting the tank run dry could leave the satellite stranded, with no fuel left to evade other craft or move to a safe orbit.

An array of capacitance sensors

The new fuel gauge, which was developed by NASA technology transfer manager Manohar Deshpande, relies on a 3D imaging technique called electrical capacitance volume tomography (ECVT). Tomography in general is a way of imaging the internal structure of an object without damaging it; familiar examples include the magnetic resonance imaging (MRI), positron emission tomography (PET) and X-ray tomography routinely employed in hospitals.

ECVT is a more recent variant, and it uses an array of sensors that emit electromagnetic waves. These waves can be detected by the other sensors in the array, and how well they are transmitted depends on the capacitance of whatever lies between the sensors. If there is nothing there, transmission will be high. However, if an object is present, transmission will drop since the object will absorb some of the electromagnetic waves. By placing these sensors around a container, and measuring the signal at many locations, it is therefore possible to build up a 3D picture of the objects inside the container.

Sensor fabrication

To make their ECVT sensors, Dagalakis and colleagues used soft lithography to print wax-coated solid inks onto paper-thin laminated copper sheets. They then etched the copper strips to remove the inks and form the patterns of the sensors and their electrical connections.

Prototype fuel tank — The interior of the prototype fuel tank. (Credit: N Hanacek/NIST)

Dagalakis notes that because the sensors can be made using well-known MEMS fabrication techniques, the dimensions of the electro-capacitance plate arrays, their gaps and shielding strips can be set with a high degree of accuracy. These fabrication techniques also eliminate the need to solder electrical wires on the plates and strips and route wires through the sensor arrays and tank structures. Finally, since the strips are flexible, they can be applied to the interior of an egg-shaped vessel – like a spacecraft fuel tank.

3D image of fuel content

Many liquid propellants (including liquid hydrogen and hydrazine) are highly flammable in the Earth’s atmosphere. As a safer alternative, the researchers tested their sensor array using a fuel substitute that has a dielectric constant similar to that of real spacecraft propellant but is stable in air.

The researchers placed their sensors around a test tank (a miniature version of a real NASA fuel tank) and measured the difference in transmission of every possible sensor pair in their array. By combining these measurements, they determined where the tank contained fuel and where it did not, gradually building up a 3D image of the fuel content along the length of the tank. Their reconstructed images showed a good match with the real shapes of the liquid inside the tank.

Space simulations

To better understand how this system would perform in space, the NIST team suspended a fluid-filled balloon inside the test tank, mimicking a liquid droplet in microgravity. They then input the resulting capacitance data into a software program to produce a series of 2D images, which they subsequently compiled to produce a 3D rendition of the balloon. The balloon’s measured diameter differed by less than 6% relative to its actual diameter.

As well as gauging fuel, the researchers say the new ECVT sensor could help overcome other problems related to liquids in space. For example, Deshpande suggests that it might be used to continuously monitor fluid flow in the many pipes aboard the International Space Station and study how sloshing fluids can alter the trajectory of spacecraft and satellites.

The researchers, who report their experiments in the Journal of Spacecraft and Rockets, plan to further test and explore various image reconstruction techniques. They are also studying ways to incorporate their sensors into new generations of long-distance spacecraft.

“The flexible and inexpensive design and fabrication of these sensors may allow several of them to be used in a spacecraft or satellite fuel tank working independently to create a composite image of the total fuel volume,” Dagalakis tells Physics World. “These sensors could have different sizes and shapes for covering the surface of the tank, any internal piping, fuel feeding pipes and fuel concentration structures.”

Sound waves in fermionic superfluid are studied in a ‘beautiful’ experiment

The acoustic properties of an ultracold fermion gas have been measured either side of the superfluid transition temperature in an experiment that has been described as “near perfect” and “beautiful”. The results could have significant implications for understanding everything from superconductors to the aftermath of the Big Bang.

Superfluidity occurs at very low temperatures when bosons such as helium-4 form a single, macroscopic quantum ground state. As well as being able to flow indefinitely without losing kinetic energy, a superfluid can famously climb uphill over a barrier to reach an energy minimum. Some fermions such as helium-3 can also form a superfluid by first pairing up to form bosons.

In helium-3, the interactions between atoms are relatively weak. However, strongly interacting Fermi gases, in which the mean free path of the particles is barely longer than the spacing between them, can also become superfluids. These systems display markedly different properties from superfluid helium-3 and cannot be described by the standard theory of superfluidity developed by the Soviet physicist and Nobel laureate Lev Landau.

Extremely useful

Despite their strong interactions, these superfluids have lower viscosities than helium-3. “Landau’s paradigm just doesn’t work,” says Martin Zwierlein of Massachusetts Institute of Technology. Finding a theory that does, however, could be extremely useful because strongly interacting Fermi gases are widespread in physics. “Take, for example, the high temperature cuprate superconductors, in which electrons interact very strongly,” explains Zwierlein. “The resistance of these materials is very difficult or impossible to calculate theoretically, we do not understand these materials.”

One way to investigate any unfamiliar material, explains Zwierlein, is to tap it and listen to the resulting sound waves. “Hilariously, this had not been done,” he says, “because for 25 years we had been using focused laser traps that were grabbing onto the atoms, creating a very inhomogeneous soup which had high density in the centre and low density at the edges – it was not nice for such an experiment.”

In the new research, Zwierlein and colleagues built a “box trap” using three laser beams. “We shape light, roughly into the shape of a Coke can, and as this light is repulsive, whenever atoms hit the walls they bounce back into the box,” he says.

First, the researchers measured the speeds at which various acoustic waves propagated through the gas in both the normal and superfluid regimes by modulating the intensity of one of the trapping lasers. They found that sound travelled at approximately the same speed, and with the same dispersion, regardless of the material’s state. The measurement enables them to predict the speed of sound in neutron stars, which are also thought to comprise strongly interacting Fermi gas, albeit with 25 orders of magnitude higher density.

“Quantum amount of friction”

Next, the researchers measured the diffusivity of the material, or how well it damps sound waves. This quantity, which has never previously been measured in a strongly interacting Fermi gas, gives crucial information about the material: “Total sound diffusivity has two components to it,” explains Zwierlein, “viscosity and thermal conductivity.” Once again, the researchers found that there was no dramatic discontinuity in sound diffusivity at the superfluid transition, but instead it reached the lowest amount permitted: “Even in the superfluid, you still have a quantum amount of friction,” says Zwierlein, “That is something I don’t think even many experts in my field realized.”

The team is now aiming to measure the thermal conductivity of the material directly, as well as looking into a bizarre phenomenon called “second sound” which occurs only in the superfluid phase. Moreover, they anticipate that their results may provide fertile ground not only for those studying high temperature superconductors and neutron stars but even for cosmologists. This is because a split-second after the Big Bang, the universe is thought to have comprised a strongly interacting quark-gluon plasma: “It turns out people predict very similar diffusivities to what we have if you replace the masses we have with the energies in the quark-gluon plasma,” says Zwierlein.

Joseph Thywissen of the University of Toronto in Canada praises the high quality of the experiment: “It’s a place in which new innovations in theory can be tested,” he says, “You can’t hide behind a messy experiment or imperfections, because the experiment is near perfect…You’ve got to get the number right if your theory is right.”

John Thomas of North Carolina State University agrees, adding “This new experiment on sound diffusivity is absolutely beautiful.” Thomas’s own group have measured the viscosity dropping to zero below the superfluid transition temperature using a different technique – which is inconsistent with Zwierlein and colleagues’ results. “There’s several different issues in this that make it unclear who’s right and what’s really going on,” he says: “It’s not a trivial problem.”

The research is described in Science.

Quantum weirdness, smart speakers, festive books: the December 2020 issue of Physics World is now out

Cover of Physics World December 2020 edition — Sounds good: the physics of smart speakers is the cover feature of the December 2020 edition of *Physics World*

“Alexa, play some Christmas music.”

“OK Google, turn on the fairy lights.”

“Hey Siri, how long do you need to cook a turkey?”

This festive season, we’ll undoubtedly be chatting to our smart speakers like they’re another member of the family, and every time, the disembodied response will be almost instantaneous.

It’s all thanks to some amazingly accurate voice-recognition technology based on ultrasensitive acoustic sensors and sophisticated machine-learning algorithms that can interpret our speech, as Pip Knight from the University of Cambridge, UK, explains in the cover feature of the December 2020 edition of Physics World magazine.

And good news for all print readers: as we promised in May, this will be the first issue of Physics World to be sent to readers in an environmentally friendly paper wrapper.

If you’re a member of the Institute of Physics, you can read the whole of Physics World magazine every month via our digital apps for iOS, Android and Web browsers. Let us know what you think about the issue on Twitter, Facebook or by e-mailing us at pwld@ioppublishing.org.

For the record, here’s a run-down of what else is in the issue.

• Heavy elements cause hefty debate – Were heavy elements produced by colliding neutron stars or supernovae? Keith Cooper tunes in to a new dispute

• ‘Delight’ as Biden elected US president – Joe Biden has already begun to initiate a science-driven agenda but faces challenges to unite a divided Congress, as Peter Gwynne reports

• Science must listen to opposing views –As the astronomy community celebrates this year’s Nobel Prize for Physics, Pruthvi Mehta says that the ongoing controversy over the Thirty Meter Telescope continues to stain the field

• Hot time for hard disks – James McKenzie looks at the power of incremental improvement andthe commercial success of magnetic‑recording technology

• Madness in the method – A new book suggests that traditional notions about “the scientific method” are flawed and misleading, as Robert P Crease discovers

• ‘Smart speaker, tell me about your acoustic sensor’ – Smart speakers that can register our everyday commands have become commonplace in homes around the world. While the acoustics sensors inside these devices are far flung from the first microphone, the technology is still evolving, as Pip Knight explains

• Thirty years of ‘against measurement’ Despite its many successes, physicists are still struggling to nail down a coherent interpretation of quantum mechanics, as it best represents “reality”. Jim Baggott explores the arguments first put forth by John Bell three decades ago, and looks at theoretical and experimental evidence accumulated since

• Following the first stars – Ian Randall reviews First Light: Switching on Stars at the Dawn of Time by Emma Chapman

• A universal theory of matter and mind – Tushna Commissariat reviews Synchronicity: the Epic Quest to Understand the Quantum Nature of Cause and Effect by Paul Halpern

• Ambassador for the Space Age – Andrew Glester reviews Not Necessarily Rocket Science: a Beginner’s Guide to Life in the Space Age by Kellie Gerardo

• Little book, big science – David Appell reviews The Little Book of Cosmology by Lyman Page 2020 Princeton University Press

• On the road less travelled – Postdoc Brooke Russell and graduate student Tamia Williams share their stories of being #BlackInPhysics, from the importance of finding your community and creating a good mentoring environment, to having the determination to succeed

• Ask me Anything – Careers advice from Carol Marsh, deputy head of electronics engineering at aerospace engineering company Leonardo who recently received an OBE for services to diversity and inclusion in electronics engineering.

• Resolving the squoon – Bradford physics teacher Nicholas Porter estimates the physical believability of Julia Donaldson’s planetary satellite in The Smeds and the Smoos

Quantum tunnelling video bags teen $250,000 scholarship, how to win a horse race

The Canadian teenager Maryam Tsegaye has bagged a total of $400,000 in prizes for making the above video about quantum tunnelling. Tsegaye, 17, is winner of the 2020 Breakthrough Junior Challenge, which was founded by the billionaires Yuri and Julia Milner. Tsegaye is a student at École McTavish Public High School in Fort MacMurray and her teacher Katherine Vladicka-Davies will get $50,000 of that prize money. The school will get $100,000 for a new science lab and the remaining $250,000 goes to Tsegaye as a college scholarship.

“Maryam’s video is a prime example of how to cleverly simplify a complex idea, and she provided a remarkable explanation of quantum tunnelling,” says Scott Kelly, a retired NASA astronaut who was a prize judge.

Tsegaye says the win is a “life-changing moment for me, and presents so many new opportunities that nothing will be the same from now on”.

Enough left in the tank

What is the ideal strategy to win a short-distance horse race – start off slow to conserve energy or go out hard and hope you have enough left in the tank for a final kick? Mathematicians Quentin Mercier and Amandine Aftalion at the School for Advanced Studies in the Social Sciences (EHESS) in Paris have now attempted to answer that question by analysing the mathematics behind thoroughbred horseracing.

They used a GPS tracking tool placed under the jockey’s saddles at the Chantilly racetracks north of Paris to create a model of winning strategies at three different lengths on a looped track: 1300 m, 1900 m and 2100 m. The model even considered the aerobic and anaerobic energy of the horses as well as the slope and bend of the track. Rather than holding the horses back for a strong finish, the researchers found that a strong start is a better strategy but keeping just in enough in reserve to provide a final kick to the finish line.

“Information on a horse speed, endurance or running economy coupled with simulations can help to predict how a horse profile is adapted to some distances to run,” the authors write, adding that “to maximize an individual horse’s potential for winning, it should be entered in races appropriate for its racing ability”.

Their paper in PLOS One is called “Optimal speed in Thoroughbred horse racing”.

Chemical precursors to life could form in dark interstellar clouds

Some key molecular building blocks of life could have been created far earlier on in the formation of the solar system than previously thought. Experiments and simulations done by Sergio Ioppolo at Queen Mary University of London and an international team have revealed how simple amino acids may have emerged via reactions on the surfaces of cold interstellar dust grains, long before the Sun first formed. The discovery could transform our understanding of how life-forming compounds first arrived on the primordial Earth.

Ice contained within comets is thought to be some of the oldest and most pristine material in the solar system. By determining its composition, scientists can glimpse conditions present when comets first formed, alongside the Sun and the planets.

In 2014, the ESA’s Rosetta probe arrived at the comet 67P/Churyumov-Gerasimenko and measured the chemical makeup of comet ice for the first time. Rosetta found evidence for the amino acid glycine and its precursor methylamine in 67P’s coma of sublimated ice. Both molecules are key building blocks of life on Earth.

Ancient molecules

Further investigation suggested that the molecules had become embedded in the pristine water ice coating the surfaces of dust particles ejected from 67P, and had never been substantially altered by heat or liquid water at any point in the comet’s history.

At the time, modelling suggested that glycine and methylamine could have been created in the ice by exposure to radiation including cosmic rays and ultraviolet photons. Now, however, Ioppolo’s team argue that this radiation would damage complex molecules after producing them.

In their study, the researchers looked at whether the molecules could have been made by “non-energetic” reactions that occurred within the dense, dark dust clouds that are characteristic of early star formation. Through lab-based experiments, they recreated these conditions using bare dust grains coated in a water-rich ice layer, which they released inside an ultra-high vacuum chamber.

Intermediate radicals

Even in such low-energy conditions, the chemistry observed by the team was surprisingly rich. Within the ice a wealth of non-energetic reactions involving atoms and free radicals was observed. This produced species including methane, ammonia, and carbon dioxide; as well as the intermediate radicals required to produce glycine and methylamine.

The team then investigated these processes further using astrochemical models. This involved using laboratory data as a basis to extrapolate the chemical reactions which would likely take place in the next million years. They calculated that glycine likely becomes abundant during the latter few 100,000s of years of evolution, as interstellar gas densities increase substantially.

Once formed, the glycine could then become a precursor to more complex amino acids, as functional groups are added to its backbone. Eventually, ice-coated dust grains would coalesce into planetesimals like comets – which can deliver complex biological molecules to newly forming planets.

The research is described in Nature Astronomy.

Dramatic footage emerges of Arecibo Observatory collapse

The Arecibo Observatory and the National Science Foundation (NSF) have released shocking footage of the moment earlier this week when the 900 tonne platform collapsed onto the 305 m-wide dish.

The video was captured by a drone that was filming as disaster struck. In a briefing along with the release of the footage, the NSF said it is now working to mitigate environmental issues and finding a way to support the scientific community.

“We recognise the significance of this loss to Puerto Rico and to so many that have called Arecibo their home whether for years or a week,” noted Ashley Zauderer, NSF programme director for Arecibo. “It inspired schoolchildren and visitors, and it inspired scientific discovery.”

‘Magnetic illusion’ can create magnetic fields at a distance

Physicists in the UK and Spain claim that they have found a way to generate and manipulate magnetic fields at a distance. This opens up the possibility of projecting magnetic fields into inaccessible spaces and enables the remote cancellation of magnetic sources, the researchers say. One application of this technique could be improving the control of magnetic microbots and nanoparticles within the human body, for medical applications such as drug delivery and magnetic hyperthermia therapies.

In recent years, metamaterials have enabled scientists to manipulate magnetic fields in unexpected ways, such as creating a magnetic cloak that can make an object magnetically undetectable and magnetic wormholes, which transport a magnetic field from one point in space to another.

In this latest work, published in Physical Review Letters, Rosa Mach-Batlle at the Universitat Autònoma de Barcelona and her colleagues wanted to see if they could generate a magnetic field that appears in free space, at a distance from its source.

Mach-Batlle tells Physics World that theory showed that to achieve this, they would need a magnetic material with negative permeability. Magnetic permeability is the ability of a material to acquire magnetization in magnetic fields. But there are no natural materials with negative permeability values.

Previously, the researchers had shown that a metamaterial with negative-permeability can be created by applying a precise arrangements of currents to the material. This time, their theory showed that a long cylindrical tube with a magnetic permeability of −1 would act as a lens for electromagnetic waves. If this cylinder was then placed around a magnetic source, the distribution of the magnetic field outside this shell would appear as if it had been created by another magnetic source – a kind of magnetic illusion – at a distance from the cylinder.

To create the metamaterial, the physicists used a 400 mm-long cylinder with a 40 mm radius. With an arrangement of 20 wires, they controlled the surface current densities on its internal and external surfaces. A wire running through the centre of the tube generated the magnetic field.

Calculated magnetic field distribution created by 20 current-carrying wires surrounding a centre wire (yellow dots) forming the replicator. The magnetic source created at a distance is seen in the shaded rectangle. (Courtesy: *Phys. Rev. Lett.* 10.1103/PhysRevLett.125.177204)

The researchers showed that with this set up they could create magnetic sources at a distance from the cylinder. They also demonstrated that the metamaterial can be used to remotely cancel another magnetic field, by setting up a wire with a current flowing through it at the point where they expected the projected magnetic field to appear. The magnetic fields generated by the wire and the metamaterial cancelled each other out. This technique could be used, the team say, to remotely cancel magnetic sources in inaccessible spaces, such as within a wall or inside a human body.

Mach-Batlle tells Physics World that it is worth noting that the magnetic field is not cancelled in all of the space. “There is a cylindrical region between our metamaterial and the source that we cancel,” she explains. “In this small region, the field is not cancelled, but in the rest of the space we would be cancelling the field of the source that we chose to cancel.”

The main practical application for this technique, Mach-Batlle believes, is not in cancelling magnetic sources, but in being able to create the illusion of a magnetic field at a distance inside inaccessible spaces. This could have important implications in medicine, as it could be possible to project a magnetic source inside the body.

If you create the illusion of a magnetic source inside the body, Mach-Batlle explains, you get a much stronger field compared with having the sources outside of the body. This could be useful for the control and manipulation of nanoparticles for drug delivery and nanorobots that could be used for various types of surgery.

“Also, in neuroscience, we think that it can have implications for transcranial magnetic simulation,” Mach-Batlle says. This technique for generating electric current at a specific area of the brain is an evolving area of research that has shown diagnostic and therapeutic potential in a number of neurological diseases and mental health conditions.

Quantum advantage demonstrated using Gaussian boson sampling

A optical circuit has performed a quantum computation called “Gaussian boson sampling” (GBS) 100 trillion times faster than a supercomputer could, according to researchers in China. This feat was achieved by Jian-Wei Pan and Chao-Yang Lu at the University of Science and Technology of China in Hefei, and colleagues. Although GBS is devised to show that a quantum computation can be done much faster than the same calculation on a conventional computer – a capability called quantum advantage – it may also have specialized practical applications.

Boson sampling is a way of computing the output of a linear optical circuit that has multiple inputs and multiple outputs. Single photons enter the circuit in parallel and encounter optical components such as beam splitters. Due to their bosonic nature, if two photons arrive at a beam splitter at the same time, they will both follow the same path. This property makes it extremely difficult to use a conventional computer to calculate the output of the circuit even for modest numbers of input photons and output channels. While boson sampling is difficult to do and requires state-of-the art quantum optics, it should vastly outperform even the most powerful supercomputers

A Boson sampling circuit can be thought of as a matrix that makes a transformation of the input photons. Calculating the output involves working out the “permanent” of the matrix, which is related to the determinant of the matrix but is much more difficult to calculate. Boson sampling determines the permanent by sending groups of single photons into the optical circuit and measuring the output. The number of photons is less than the number of output modes of the circuit – so three photons could be sent into a six-mode circuit, for example.

Single mode squeezed states

In this latest research, the team used a related technique called Gaussian boson sampling (GBS), in which single mode squeezed states of light are used in place of single photons. Instead of determining the permanent of the circuit, Gaussian boson sampling gives a similar quantity that is also extremely difficult to compute using a conventional computer.

According to Pan, GBS offers two important advantages over the single-photon technique. First is that the photon generation rate is much higher for GBS than it is for single-photon boson sampling, and second there are “many proposals for practical applications based on [GBS]”.

The team’s optical circuit has 100 inputs and 100 outputs and comprises 300 beam splitters and 75 mirrors that are arranged in a random manner. The system is fully connected, so a photon at any input port can emerge from any of the output ports.

GBS took about 200 s to make the desired calculation, whereas the team estimate that China’s fastest supercomputer Sunway TaihuLight would take 2.5 billion years to do the calculation.

“Important milestone”

“This experiment is definitely an important milestone for quantum simulations based on linear optical systems,” says Christine Silberhorn at Paderborn University in Germany – who along with colleagues first proposed GBS in 2017. She points out that scaling-up the system to its 100×100 size would have been very challenging.

Ian Walmsley at Imperial College London agrees, adding that the team has made a “heroic effort” at “preparing quantum states that are entirely indistinguishable, and making sure the photons aren’t lost”. However, he points out that use of bulk optics rather than integrated optics could make it difficult to further scale-up the system.

Undaunted, Lu says that the team has made a “considerable improvement of the efficiency of the quantum light sources,” which he says should enable a 144×144 version of the experiment. Looking further ahead he says, “In 2021, we will make the GBS machine more tuneable, more compact and more stable, and look for practical applications”.

Molecular spectra

Although the current system has no practical application beyond demonstrating quantum advantage Pan says, “we are excited by the potential usefulness of boson sampling as the community has come up with many ideas”.

Walmsley adds, “There are some interesting simulation problems that might benefit from this new scale b boson sampler, including modelling of molecular spectra and vibrational dynamics. However, even those require the addition of a deterministic nonlinearity at the single-photon level in order to be able to handle real-world systems accurately.”

The research is described in Science.

A passion for nanotechnology in medicine: Black in Nanotech Week cofounder Olivia Geneus on inspiring new nanoscientists

The second week in December is Black in Nanotech Week and its co-founder Olivia Geneus is our guest in this episode of the Physics World Weekly podcast. Geneus talks to Margaret Harris about her interest in using nanotechnology to develop new ways of treating cancer, and about the need to highlight the accomplishments of Black scientists in the field of nanotechnology and inspire the next generation of nanoscientists.

We love a quiz here at Physics World, so to celebrate this week’s Materials Research Society Virtual Meeting and Exhibit we have put together a quiz about songs and bands with materials in their titles or lyrics. In the podcast, quizmaster Matin Durrani tests the musical knowledge of Tom Miller, who runs several materials-related journals at IOP Publishing.

This week we learned that the Arecibo Observatory in Puerto Rico has been destroyed as its metal platform collapsed onto its reflecting dish. We chat about the sad demise of this iconic radio telescope and its effect on Puerto Rico and the astronomy community.

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